This invention relates to a method to detect a defocused photographic image recorded on a photographic film.
When a photographic film is to be printed, it is necessary to remove such defocused images (blurred images) which are caused by improper focusing. There have been various methods for detecting such defocused photographic images in prior art.
In one method, defocused images are discriminated by detecting whether the spectral components thereof are within a predetermined domain of frequency by means of the Fourier Transform of the images or not. In this method, however, if the Fourier Transform is conducted optically, it requires a highly accurate, complex and expensive system while the Fourier Transform is conducted electrically, there are no Fourier Transform means available which are fast enough to suit for the photographic printer generally used.
In another method, photographic films are scanned linearly, the maximum density gradient within the high frequency band and the maximum density gradient of images blurred by removing the high space-frequency domain are obtained, the quotient of the two maximum density gradient is calculated. Then, the defocused images are detected by judging whether the obtained quotient is larger than the prescribed threshold value or not. (e.g. Japanese patent application Laid-open No. 70428/1978). The method, however, needs a large number of measurement points, thereby presenting difficulties in high-speed processing. Further, since the measurement of density largely depends on the scanning direction, it is difficult to obtain two-dimensional density gradients. The system is detrimental also in that it needs two circuits, i.e. a circuit to obtain the maximum value in the density gradients and the one to obtain the quotient of the two density gradients, making the system complicated.